1. Field of the Invention
The invention relates to a partial discharge charge amount (or electric charge amount in partial discharge) measuring method and a partial discharge charge amount measuring device for indexing the lifetime assessment of, e.g. electric wires such as enameled wire.
2. Description of the Related Art
For example, an electric wire such as enameled wire is formed by providing an insulation (insulating film) so as to surround a conductor. In such an electric wire, partial discharge may occur. The partial discharge occurs due to microvoids within the insulation of the electric wire or between the conductor and the insulation, where an electric field concentrates and very weak discharge occurs thereby. The partial discharge causes a deterioration of the insulation, which may result in an insulation breakdown.
Recently, some motors are driven at variable speed using an inverter power source from the viewpoint of energy saving and improvement in efficiency. A voltage applied from the inverter power source to the motor is a pulse voltage (hereinafter, referred to also as “inverter pulse voltage”). Therefore, when the motor is driven using the inverter power source, the motor is subjected to a high-voltage pulse with a short rise time (hereinafter, referred to also as “inverter surge voltage”). This means that a large electric current is fed through an electric wire (i.e. conductor) used in the motor and surge overvoltage (or inverter surge voltage) is thus applied to the electric wire. Due to the inverter surge voltage applied, a high electrical stress is applied to the electric wire. This causes an increase in electric field intensity in the voids, whereby a partial discharge may be caused in the electric wire. As a result, the insulation breakdown may occur in the electric wire.
Thus, the lifetime of electric wire is sometimes assessed so as to estimate a time to the breakdown of electric wire (i.e., a lifetime of electric wire). The lifetime of electric wire depends on the level of voltage to be applied (impressed) and the time of impressing voltage to the electric wire. Accordingly, the conventional lifetime assessment of electric wire is often carried out by using a V-t test. That is, for example, a predetermined level of inverter voltage pulse and a predetermined frequency are applied to a sample of an electric wire etc. and a time to the breakdown is measured to assess the lifetime of the electric wire (see, e.g. JP-A-2008-270227).